1. Field of the Invention
The present invention disclosed in the specification relates to a method for producing a semiconductor device having a gate electrode using a crystalline thin film semiconductor, for example, a thin film transistor (TFT). As application of the TFT, an active matrix type liquid crystal display device has been known. This display device performs a fine and high resolution display by arranging a TFT as a switching element in each of several hundred thousands or more pixels disposed at a matrix form.
2. Description of the Related Art
Recently, a transistor using a thin film semiconductor formed on a glass or quartz substrate, such as a thin film transistor (TFT) has been suggested. A thin film semiconductor having a thickness of several 100 to several 1000 .ANG. is formed on a surface of a glass substrate or a quartz substrate, and then the transistor (insulated gate field effect transistor) is formed using the thin film semiconductor.
Of such the TFT, a TFT using an amorphous silicon film and a TFT using a crystalline silicon film is used in practice. Since the TFT using the crystalline silicon film has a superior characteristic, it has a great future.
In the TFT using a crystalline silicon semiconductor, the crystalline silicon thin film is obtained by a method for thermally-annealing an amorphous silicon film, or a method for forming a crystalline silicon film directly using a vapor phase growth method. However, in order to perform the process at a low temperature, a photo-annealing for crystallizing an amorphous silicon film by irradiating an intense light such as a laser has been proposed. (for example, Japanese Patent Application Open No. 4-37144)
There are two main methods used to obtain a crystalline silicon film by photo-annealing. In a first method, photo-annealing is performed after etching a semiconductor thin film into a shape of an element to be formed. In a second method, after photo-annealing for an even (flat) film is performed, the film is etched into a shape of an element to be formed. In general, it has known that the element obtained by the first method has a superior characteristic (field effect mobility) than that obtained by the second method. This may be because in the first method, the film is contracted by photo-annealing, and thus a central portion of a pattern is stressed, thereby increasing crystallinity of the film.
However, there is a problem in this case. That is, although an initial characteristic is good, after use for a long period of time, the characteristic is deteriorated largely.
The cause for the deterioration the conventional method will be explained with FIGS. 3A to 3D. Initially, an island semiconductor region 31 of amorphous silicon having a rectangular shape 32 is formed as shown in FIG. 3A. When photo-annealing is performed, the film is contracted slightly by crystallization. A dotted line of the figure represents a size of the island semiconductor region before the photo-annealing. In this contraction process, a region 33 is formed where distortion is accumulated in an outermost portion of the island semiconductor region 31 is formed. The crystallinity of such the region 33 is not as high. (FIG. 3B)
A gate electrode 34 is sometimes formed across such an island region (FIG. 3C) FIG. 3D shows (a-b) cross section along the gate electrode, the region 33 where distortion is accumulated however, is formed under the gate electrode 34 and a gate insulating film 35. This deteriorates an interface characteristic between the region 35 and the gate insulating film. Therefore, when a voltage is applied to the gate electrode 34, charge is trapped, so that deterioration occurs by a parasitic channel or the like due to the charge trapping. (FIG. 3D)